Frequency discriminator



May 25, 1954 E. J. DRAZY FREQUENCY DISCRIMINATOR Filed Oct. 25, 1.949

FIG. 5

.02 '04 EERCENTAGE DEVIAT/ON 0F INPUT FROM REFERENCE mzousn/cr /N|/ENTOR E J DRAZY Arrom/Ev Patented May 25, 1954 UN ITE D PATENT FREQUENCYDISCRHVIINATOR: r

Elbert J. Drazy, East Orange, N. .L, assignor to, Bell TelephoneLaboratories, Incorporated, New'York', N. Y.', a corporation of New YorkApplication'0ctober25, 1949, Serial No. 123,323

13 Claims.

This invention relatesto: circuitswhich are responsive to frequencyvariation, and more particularly to a frequency discriminator adaptedfor use in the microwave range.

An object of the invention-is to -produce an output voltage which isproportional to, and the sign of which is determinedbmthe deviation ofthe input signal from a reference frequency.

A further object is to increase the sensitivity of a frequencydiscriminator.

Wave transmission systems often require a devicewhich will developacross its output terminals a voltage the magnitude of which issubstantially proportional-to the deviation from the nominal orreference frequency of the inputsignal, and the sign of which dependsupon whether the frequency is higher or lower than. the reference. Sucha device, called afrequency discriminator, finds application, forexample, as a component in circuits for oscillator frequencystabilization or as a demodulator for frequencymodulated signals.

Thefrequency discriminator of the present invention makes use of hollowwave-guide structures and is particularly adapted for operation in themicrowave frequency range. Inthe embodiments shown, by way of exampleonly, the discriminator comprises a main wave-guide, a cavity resonatorcoupled to one'endthereof, two auxiliary wave guides directionallycoupled to opposite sidesof the main guide, andmeans at one end of eachauxiliaryguide for deriving a unidirectional potential proportional tothe microwave energy therein. The auxiliaryguides are short-circuited attheir other ends at points so chosen that the differencebetween thelength of the electrical path from the input end of the main guide tothe shorted end of one of the auxiliary guides and-the length of thepath from the same point to the shorted end of the other auxiliary guideis approximately equal to an odd integral number of quarterwavelengths,and that each of these. lengths differs from the length of themain guideby approximately an odd integral number of eighth wavelengths, at theresonant frequency of the resonator. These two potentials are combinedin the series-opposing relationship to provide a voltage which isproportional to the deviation 'fromfthe resonant frequency oftheresonator, and the sign'of which is positive foran increase in'frequencyand negative for a decrease.- The-resonator, which is resonant at thereference frequency, contributes materiallyto the sensitivity of thediscriminator, since its. 'reactance' changes sign at its-resonantfrequency and has asteep-slope on either side thereof. The microwaveenergy detectors may, for example, be crystals, thermistors,.or.thermionic diodes.

The nature of the invention will be more fully understood from thefollowing detaileddescription and by reference to theaccompanyingdrawings, of which Fig. l is a longitudinal sectional View of afrequencydiscrimina-tor in accordance with. the invention;

Fig. 2 is an end view'of-the discriminator of Fig. 1 as seen from theright Fig. 3 shows the portion of Fig.- 1 to the left of the line XX andillustrates a modification in which the crystal detectors are-replacedby thermistors; I

Fig. 4 is similar to Fig. Band showsanother modification in which thecrystals are replaced by thermionic diodes; and-- Fig. 5 is a typicaldiscrimination character istic of the discriminator showing the ratio ofoutput to maximum voltage, plotted-against the deviation of the inputfrequency from the-:reference frequency.

The frequency discriminator shown in: Figs.-

1 and 2 comprises a hollow main-waveguide ID of rectangularcross-sectiom a cavity res onator ll, two aperiodic rectangularauxiliary wave guides l2 and I3, anddetecting means 14 and I5 forderiving unidirectional potentials proportional to the microwaveenergywithin the auxiliary guides 12 and I3; Microwave energy of nominalor reference frequency from some suitable source is fed into the leftendaof the main guide ID, as indicated by-thearrow; The

unidirectional output voltage appears at the terminals l6 and I1 and ismeasured-by means of a direct-current voltmeter or other appro-- priateindicating device.

The cavity resonator II is electromagnetically coupled to the right-handend of the main guide H] by means of the orifice I8 and is adjusted-toresonate at the frequency f by means of the slidable piston |9.-Formaximum discrimination the resonator II should match the guide H) inimpedance at the frequency f.'. The auxiliary guides l2 and I3adjoin-the opposite wider sides 21 and 22 of the main guide-wandare'eleetromagnetical-ly coupled-thereto by the coupling orifices 23,24, 25', and 26, 21-,- 28, respectively. These orifices are so spacedand the areas thereof are so chosen that the couplings between the mainguide land the auxiliary guides l2" and l3 are equal and aredirectionally selective;

that is, left -to right signal propagation in the main guide ID willinduce left-to-right signal propagation in the auxiliary guides I2 andI3, but will not produce appreciable right-to-left signal propagationtherein.

The right-hand ends of the auxiliary guides I2 and I3 are closed by theslidable short-circuiting istons 30 and 3|, respectively. These pistonsare so positioned that the difference between the length of theelectrical path from the input end 32 of the main guide ID to one of thepistons 30 and the length of the electrical path from the input end 32to the other piston 3I is approximately equal to an odd integral numberof quarter wavelengths and, in addition, that each of these lengthsdiffers from the length of the main guide III by approximately an oddintegral number of eighth wavelengths, at the frequency ,1. Therefore,the distance S between the planes of the inner faces of the pistons 30and 3| is made approximately equal to m/l, where x is the wavelengthwithin the guide and n is any odd integer. In Fig. l, n is unity, andthe length of the auxiliary guide I2 is longer than that of theauxiliary guide I3. Also, the distance D1 between the plane of the innerface of the piston 3I and the plane 33 of the orifice I8 is madeapproximately equal to an odd integral number of eighth wavelengths andthe distance D2 between the plane 33 and the inner face of the piston 30is made approximately equal to an odd number of eighth wavelengths.Therefore, D1 is approximately equal to m1 8 and D2 is approximatelyequal to max/8, where m1 and me are odd integral numbers and may beeither the same or different. In Fig. 1, m1 and ma are each equal tounity.

The left-hand ends of the auxiliary guides I2 and I3 are terminated indevices for detecting and rectifying the microwave energy therein. Asshown in Fig. 1, these devices are the crystal detectors I4 and I5,respectively, represented schematically. These crystals are connected inseries-opposing relationship between the output terminals I3 and I1, andare paired and individually matched in impedance to the associatedauxiliary guide at the operating frequency ,"f.

In order to prevent the escape of microwave energy from the auxiliaryguides I2 and I3 a quarter-wave choke is provided between each of thedetectors I 4, I5 and the associated output terminal. This comprises acylindrical conductive sleeve 31 which surrounds the conductor 38. Thesleeve 31 has a length approximately equal to M4, is open at the outerend, and at its inner end 39 is short-circuited and conductivelyconnected to the conductor 38. 1

When the frequency discriminator is used in applications such asoscillator stabilization, where only comparatively slow drifts offrequency are to be corrected, the crystal detectors I4 and I5 can bereplaced by the pair of thermistors 40 and 4|, the equal resistances 4Band 47, and the battery 48, as shown in Fig. 3. Thermistors have theadvantage that their impedance match can be maintained more easily overwider bands than is the case with crystals. Also, in some applications,the crystals I4 and I5 can advantageously be replaced by the pair ofthermionic diodes 42 and 43, as shown in Fig. 4. The sources 44 and 4-5supply the cathode current for the diodes 42 and 43, respectively.

An explanation of the operation of the frequency discriminator shown inFigs. 1 and 2 will now be given. When a microwave signal, hereaftercalled the incident signal, traveling from left to right in the mainguide I0, reaches the coupling orifices 23 to 28 it induces in theauxiliary guides I2 and I3 equal signals proportional to, andpropagating in the same direction as, the incident signal. When theseinduced signals in the auxiliary guides I2 and I3 reach the righthandends thereof they arereflected by the shortcircuiting pistons 30 and 3|,reverse their direction of propagation, and travel to the left ends ofthe auxiliary guides where they aid in exciting the crystal detectors I4and I5.

The portion of the incident signal in the main guide In which passes theorifices 23 to 28 continues traveling to the right until it reaches theorifice I8 of the cavity resonator II, where it is partially reflected.The part which is reflected depends, both in magnitude and in phase,upon the deviation of the incident signal from the reference frequencyf, and increases in magnitude as the deviation increases. This reflectedsignal in the main guide I0 propagates from right to left and, as itpasses the orifices 23 to 28, induces in the auxiliary guides I2 and I3equal signals proportional in magnitude to that of the reflected signalin the main guide II].

There thus exist in each of the auxiliary guides I2 and I3 twocomponents of signal, one induced by the incident signal in the mainguide I0 and the other induced by the reflected signal in the mainguide. When the incident signal is of frequency ,7 there is a minimumreflection of it at the orifice I8 because the resonator I I is resonantat this frequency and presents a substantially nonreactive impedancewhich referably matches the characteristic impedance of the guide ID.Due to the positions of the pistons 30 and 3I with respect to each otherand with respect to the plane 33 of the orifice I8, the two signalcomponents in each of the auxiliary guides I2 and I3 are in quadrature,and their vector sums are equal. Therefore, the rectified voltagesappearing across each of the detectors I4 and I5 are equal, and sincethese are connected series opposing the output voltage at the terminalsI6 and I! will be zero.

The reactive component of the impedance of the resonator II, as viewedat the orifice I8, passes through zero at the frequency ,f and has anegative slope on each side thereof. Therefore, this component increaseswith the frequency deviation and is capacitive above 1 and inductivebelow 1. This reactance, in turn, causes a phase shift in the reflectedsignal in the main guide In which increases with the frequency deviationand is leading below 1 and lagging above 1. When this phase shift islagging, the angle between the two signal components in the auxiliaryguide l2 becomes acute and their vector sum is increased, While theangle between the signal components in the auxiliary guide I3 becomesobtuse and their vector sum is decreased. As a result, the detector I4is more strongly excited than the detector I5 and, because of theseries-opposing connection of the detectors, a net positive potential isobtained between the output terminals I6 and I1. This positive potentialincreases in magnitude as the frequency of the incident signal increasesfrom I.

On the other hand, at frequencies of the incident signal below 1, theleading phase shift produces an obtuse angle between the signalcomponents in the auxiliary guide I2 and an acute angle between thesignal components in the auxiliary guide I3. As a result, the detectorI5 is more strongly excited than detector I4 and there appears at theoutput terminals l6 and H a net negative potential which increases asthe signal frequency decreases from f.

Fig. 5 shows a typical discrimination characteristic for the frequencydiscriminator of Figs. 1 and 2. The ratio of the rectified outputvoltage to the maximum voltage appearing at the terminals l6 and I1 isplotted against the percentage deviation of the input or incident signalfrom the nominal or reference frequency ,f. This characteristic appliesto a discriminator in which the impedance of the cavity resonator IIsubstantially matches the characteristic impedance of the main waveguide I 0 at the frequency f, and the resonator II has a Q of 5000. Thefactor Q may be defined as the ratio of the energy stored to the energydissipated per cycle. It is seen that at the frequency f, where thedeviation is zero, the output voltage is also zero but increasesnegatively for the deviations below I, and positively for frequenciesabove 1. Over the deviation range from 0.01 to +0.01 per cent thecharacteristic is quite steep and is substantially linear. The slope inthis region, and therefore the sharpness of discrimination, may beincreased by increasing the Q of the resonator ll.

Many modifications of the frequency discriminators herein disclosed willreadily occur to persons skilled in the art. For example, in thestructure shown in Figs. 1 and 2, either the piston 30 or the piston 3|may be moved either to the right or to the left by any small integralnumber of half wavelengths without substantially affecting the operationof the discriminator. Also, the orifice [8 may be moved either to theright or to the left by any small integral number of half wavelengthswithout substantially changing the performance. Furthermore, the orificeI8 may be moved either to the right or to the left by any small oddintegral number of quarter wavelengths with only the result that thepolarity of the output voltage at the terminals l6 and I! will bereversed. Of course, the resonator ll must be retuned to the frequency 1when the orifice I8 is moved, and neither the orifice IS, the piston 30nor the piston 3| can be moved so far to the left that it will interferewith the proper functioning of the right-hand coupling orifices 25 and28.

What is claimed is:

1. A frequency discriminator comprising a main wave guide adapted at oneend for the reception of electromagnetic waves of nominal frequency f, acavity resonator resonant at approximately the frequency f andapproximately matching said main guide in impedance at said frequency,means for electromagnetically coupling said resonator to the other endof said guide, two aperiodic auxiliary wave guides directionally coupledto opposite sides of said main guide, means for short-circuiting theends of said auxiliary guides towards which said waves are directionallypropagated, two detectors located, respectively, at the other ends ofsaid auxiliary guides for deriving a unidirectional potentialproportional to the high frequency power therein, each of said detectingmeans being approximately matched in impedance to the associatedauxiliary guide, and means for connecting said detectors in series, thedifference between the length of the electrical path from said one endof said main guide to the short-circuited end of one of said auxiliaryguides and the length of the electrical path from said one end of saidmain guide to the short-circuited end of the other of said auxiliaryguides being approximately equal to an odd integral number of quarterwavelengths, and each of said lengths differing from the electricallength of said main guide by approximately an odd integral number ofeighth wavelengths, at the frequency J.

2. A frequency discriminator in accordance with claim 1 in which saiddetectors are paired at the frequency f.

3. A frequency discriminator in accordance with claim 1 in which each ofsaid detectors comprises a crystal.

4. A frequency discriminator in accordance with claim 1 in which each ofsaid detectors comprises a thermistor.

5. A frequency discriminator in accordance with claim 1 in which each ofsaid detectors comprises a thermionic diode.

6. A frequency discriminator in accordance with claim 1 which includesmeans for adjusting the lengths of said auxiliary wave guides.

7. A frequency discriminator in accordance with claim 1 which includesmeans for adjusting the resonant frequency of said resonator.

8. A frequency discriminator comprising a main wave guide adapted at oneend for the reception of electromagnetic waves of a selected nominalfrequency, a cavity resonator resonant at approximately said frequencyand approximately matching said main guide in impedance at saidfrequency, an orifice coupling said resonator to the other end of saidguide, two aperiodic auxiliary wave guides directionally coupled toopposite sides of said main guide, means for short-circuiting the end ofone of said auxiliary guides towards which said waves are directionallypropagated at a point A/8 to one side of the plane of said orifice,means for short-circuiting the end of the other of said auxiliary guidestowards which said waves are directionally propagated at a point M8 tothe other side of said plane, two detectors located, respectively, atthe other ends of said auxiliary guides, each of said detectors beingapproximately matched in impedance to the associated auxiliary guide,and means for combining the output voltage of said detectors inseries-opposing relationship, the longitudinal axes of said wave guidesbeing approximately parallel and A being the wavelength within theauxiliary guide under consideration at approximately said frequency.

9. A frequency discriminator in accordance with claim 8 in which each ofsaid detectors comprises a crystal.

10. A frequency discriminator with claim 8 in which each of comprises athermistor.

11. A frequency discriminator with claim 8 in which each of comprises athermionic diode.

12. A frequency discriminator in accordance with claim 8 in which saidshort-circuiting means are adjustable along said auxiliary guides.

13. A frequency discriminator in accordance with claim 8 which includesmeans for adjustthe resonant frequency of said resonator.

References Cited in the file of this patent UNITED STATES PATENTS inaccordance said detectors in accordance said detectors Number Name Date2,153,728 Southworth Apr. 11, 1939 2,413,939 Benware Jan. 1 47 2,420,892McClellan May 20, 1947 2,423,390 Korman July 1947 2,562,281 Mumford July31, 1

